You'll make me cry," Ruth Negga sighs. There is a pantomime frown before those moonbeam eyes re-emerge smiling.

I had sought to begin our chat with something light and easy about Negga being finally home after seven long months of promotional duty. Seven months of hotel rooms "exactly" like the one we are seated in today in the Westbury, and seven months of questions, questions and more questions.

This is her first proper interview junket of this campaign back in the old sod, however. After we're done, she plans to brave the Grafton Street shopping hordes and meet some friends for a catch-up drink.

"At the risk of being cheesy, it's very nice to be home," she says, cosying into her chair. "I find it a bit like breathing out and relaxing here. There is the relief of being understood when you come back. There's one word, I find: 'Water'. Nowhere but here will anybody understand me when I say it. Not in America. Not even in England. It's that lack of having to repeat oneself, and I know that sounds really petty and tedious. Irish people are also quite sensitive about imitations of our accent. I think it's really bad that people feel they can do it when they meet us. I don't find it funny impersonating anyone's accent…"

This is some weeks before Meryl Streep will use Negga to help make a point in her recent and now-famous Golden Globe speech about celebrating Hollywood's diversity. Not only will Streep nearly call Negga a Londoner (more of which later), but she will correct herself with a stage-Irish accent. Whether Negga picked Tinseltown's grand dame up on this afterwards is not known but the following night on Jimmy Kimmel Live! she will call Streep's affectionate shout-out "a dream come true".

Be it from international press or big-screen legends, such attention tends to be par for the course when you deliver one of the most talked about performances of the year. Loving is a richly understated biopic of game-changing interracial couple Mildred and Richard Loving. The film was in development long before last year's "Oscars So White" controversy and now finds itself hitting nerves with sectors of American society that are dismayed over that nation's ugly political swing to the Right. Negga's turn as Mildred is the movie's epicentre, and with a rake of nods already collected this awards season, all eyes are now on Tuesday's Academy nominations announcement and that most glittery of nights in the Dolby Theatre a month later.

Not bad for the Limerick girl who became a mainstay of the acting circuit up in Dublin before her horizons expanded to London and the United States. Wherever she appeared - The Crucible in the Abbey, on our screens in Love/Hate, whooping ass in Preacher - nuance, poise and perspiration tagged along. Irish audiences used to her presence may baulk at the "newcomer" mantle she is being ascribed outside our shores now, but we must concede that Loving is a breakthrough role for the 35-year-old that will most likely change her life.

Directed by Jeff Nichols (Mud, Midnight Special), Loving emphasises the fact that the couple at the heart of its tale - who violated Virginia's interracial laws when they married in 1958 - were not torch bearers. They were just ordinary people who loved being together.

Neanderthal ideas that God intended the races to be kept separate by sea borders were finally disposed of in the 1967 Supreme Court case that ruled in their favour. Negga and co-star Joel Edgerton keep the wattage low and the undercurrent powerful.

She was born in Addis Ababa, where her Ethiopian father and Irish mother were a hospital doctor and nurse, respectively. At age four, Negga came to Limerick with her mother. Her father was set to follow them but was killed in a car accident when she was just seven. She remained in Ireland until a stint in South London when she was 11.

Then it was back to Dublin to train in acting at Trinity's Samuel Beckett Centre and work as a journeyman actor. Eventually, there was a full-time move to London in 2006, where she lives in Primrose Hill with partner and Preacher co-star Dominic Cooper ("It's nice having someone who understands that what you're negotiating is something very odd. I'm very lucky."). She surely considered her parents' relationship while portraying Mildred.

"I don't know if I did to a great extent," she says slowly. "Mildred reminded me a lot more of my auntie Pauline. She died last year and I was very close to her, and I did harness a lot of her. We were very much reared by the women in our family and kind of absorbed that maternal mothering way that they had about them. The countryside formed her, just like Mildred. I lovingly stole bits from her."

Loving's content has naturally led many interviewers to ask about her own background and the prejudice that she and her parents perhaps encountered ("none whatsoever") which gets understandably tiring. Even without me asking, she finds herself explaining this to me.

"I have asked my mother," she says, "and I don't think she's lying, she's quite an honest woman - they never had any problem in Ethiopia, ever. My family never expressed any wariness. That's not my narrative so I'm not going to lie and say there was. When I came home from the hospital, everyone was delighted with me!"

Promoting Loving, however, has opened her eyes to how lucky they had it. After one US screening, a girl not much older than Negga approached her, speaking about how her maternal grandfather forbid her black father from entering his home.

"And it was only after the grandfather had died that the father told the daughter because he didn't want her to think ill of her own blood. All these strange stories that are quite moving. The lovely thing is people are coming up to us wanting us to bear witness to their history."

"I get impatient with people's curiosity," she says, a little guiltily, just afterwards. "I used to find it very alienating for me. People used to look at my history like I was some sort of exotic curiosity. For me, that's my normality, and it's very isolating when people always see you as something that is 'other'."

I remark that Phil Lynott's statue is a stone's throw from the hotel. His surrounds were far less racially diverse, and his talent and charisma overshadowed that exoticism.

"Yes," she grins. "It's awful when you talk about people like they're your mate, isn't it? 'Me ol' pal Philo!' The great thing about Phil Lynott - I just totally love him to pieces - is that his blackness wasn't indistinct from his Irishness.

''They were very much the one and the same, and I always felt like that. I am over saying 'I am this' and 'I am this'. I don't think that I have to choose. People are always wanting you to choose in a very stark manner about your life. We have to afford our own selves and others more complexity than that." Negga looks unreasonably glamorous this dull winter morning, and in a certain light, she can generally come across quite demure. The truth is she's actually full of craic, and has a wonderfully childish nasal snigger that you suspect has been refined over a lifetime of mischief. She ties herself in knots recalling an interviewer who repeatedly began questions with "as a British actress" ("It's not that the idea disgusts me, it's just that it's not true!") and then became worried her protestations would label her "irate Irish actress Ruth Negga".

She is thoughtful too, riffing away on how her move to London a decade ago exposed to her just how Irish she truly was (she suffers bouts of "deep homesickness"), our relationship with land and "the mythmaking that we've internalised". She is up on everything from last year's Marriage Referendum to Waking the Feminists and Repeal the 8th, all of which she calls "moving" displays of social activism spearheaded by "ordinary people with a need, just like Richard and Mildred".

But underneath all this is a genuine shyness, the kind that struggled to adjust to phalanxes of flashbulbs when Loving debuted at Cannes last May. It is there as her voice descends to a whisper and expression becomes evasive. It is a side of her that many simply refuse to believe can exist in a "film star".

"Cannes was a major jump for me," she exhales in a flood of recall. "People are so surprised when you say you're a bit nervous about going on a chat show or whatever. 'Why not just play a character?' they say, but these are almost opposite ends of the spectrum - being oneself and being a character. One can be many things. I was a very shy child but I was also an extrovert. It's possible to inhabit those two camps, side by side. The great thing is that acting serves that purpose. Even looking into a camera at a photoshoot is so alien because you're trained not to see the camera or be aware of its presence when filming."

She suddenly reminded herself to show some good-mannered humility about her lot, as if the homeliness of her Ethiopian and Irish roots are reining in a fledgling Hollywood diva that just threatened to break out.

"But it's not something that I would complain about - it's a lovely thing - but it is just a by-product. I hate to use that word because that sounds dismissive, and I don't want to be dismissive of it - it's a very strange thing to happen. Like being on the front cover of Vogue. Like, I never even dreamed that was a possibility. I have huge respect for Vogue, huge respect for [editor] Anna Wintour. I was buying this magazine when I was a teenager.

"It's intelligent, it's high art, it's a celebration of women. And to be a brown person on a cover of something is also very cool."